Add "matchingTarget" function to Event, which is the Element that matched the CSS value selector.

tools/dom/templates/html/impl/impl_Event.darttemplate

Index: tools/dom/templates/html/impl/impl_Event.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Event.darttemplate b/tools/dom/templates/html/impl/impl_Event.darttemplate
index a3fc47f660dd5faa429cd19a30bde06f4fc51673..ef07e13c1cdca386238aa5b5f3202f5500e8ac03 100644
--- a/tools/dom/templates/html/impl/impl_Event.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Event.darttemplate
@@ -33,7 +33,30 @@ $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
     e._initEvent(name, canBubble, cancelable);
     return e;
   }
+  
+  /** The CSS selector involved with event delegation. */
+  String _selector;
 
-  $CLASSNAME._private();
+  /**
+   * A pointer to the element whose CSS selector matched within which an event
+   * was fired. If this Event was not associated with any Event delegation,
+   * accessing this value will throw an [UnsupportedError].
+   */
+  Element get matchingTarget {
+    if (_selector == null) {
+      throw new UnsupportedError('Cannot call matchingTarget if this Event did'
+          ' not arise as a result of event delegation.');
+    }
+    var currentTarget = this.currentTarget;
+    var target = this.target;
+    var matchedTarget;
+    while (matchedTarget == null && target != currentTarget && target != null) {
+      if (target.matches(_selector)) {
+        matchedTarget = target;
+      }
+      target = target.parent;
+    }
+    return matchedTarget;
+  }
 $!MEMBERS
 }